It is known to produce industrial components or articles automatically by the transformation of successive layers of raw material from a first state into a second state using computer information representing their shape. Machines using this principle, called Rapid Prototype machines, operate in most cases by repetition of a cycle comprising the following steps:
a covering phase during which the stack of layers already produced during the previous steps is covered by a layer of non-transformed raw material; PA1 a phase of partially transforming the new layer by means of a device which emits electromagnetic radiation or particles of matter in the direction of that face of the layer which is not in contact with the previous layers. PA1 a phase of transforming the raw material in at least part of a working field by means of the transformation-inducing device, PA1 a phase of covering the transformed material with untransformed material, the covering phase using at least one doctor blade set in motion in order to carry out the various covering phases, PA1 a device inducing the transformation of the raw material, PA1 optionally, a container for containing the raw material, PA1 means for moving the already transformed volumes with respect to the working field, PA1 means for carrying out the covering phases using one or more doctor blades, the doctor blade(s) being set in motion approximately parallel to the working field during the covering phases by means of guiding and driving means,
International Application PCT WO 93/25377, for example, describes a rapid prototyping machine using the action of light radiation to partially solidify successive layers of liquid resin contained in a tank. The transformation-inducing device consists of a light generator and of an optical system which makes it possible to direct the photons emitted by tip light generator onto a portion of the free surface of the liquid corresponding with the section of the component to be produced. After each transformation phase, a support integral with the layers already partially solidified is moved vertically by a motor-driven system in order to reduce the height of the component being created with respect to that of the free surface of the liquid. Very often this movement is not sufficient to cover the parts that have just been solidified with the liquid resin and to ensure sufficient flatness of the free surface of the liquid to be able to carry out the next transformation phase. This is why a mechanical component of elongate shape, called a doctor blade, the lower part of which is in contact with the resin, is moved, in an approximately horizontal movement, by a motor-driven mechanical device, to progressively sweep over the entire surface accessible to the transformation-inducing device and to obtain the desired covering and desired flatness before moving on to new transformations. All the components of the machine are controlled by a computer, using computing data which is generated (by software processing) from the initial data and which determines the shape of the components to be produced.
Document PCT WO 93/25377 proposes various embodiments of the doctor blade; in particular, in FIG. 13 of that document, a doctor blade is described which consists of a rigid support coming into contact with the resin, by means of a plurality of flexible films called moisteners. Application EP-A-0,484,182 also contains a description of a doctor blade: this doctor blade consists of two rigid supports arranged parallel and lying close to each other, to which are fixed brush elements which come into contact with the resin via their lower part.
The type of transformations carried out in the context of rapid prototyping machines is not limited to photopolymerization or photocrosslinking of liquid resins. For example, cutting sheets of paper or agglomerating powders (plastic powders, metal powders, powder blends, etc.) using a laser are techniques commonly used for producing components in rapid prototyping. Rapid prototyping processes also exist which, instead of using light, operate by the addition of material. Mention may be made, for example, of the process developed at M.I.T. and described in the work "Rapid Prototyping & Manufacturing--Fundamentals of Stereolithography (Paul F. Jacobs, published by "Society of Manufacturing Engineers", page 409), in which the local solidification of layers of ceramic powder is carried out by the addition of an aqueous binder in the form of droplets expelled by nozzles.
Despite the wide variety of materials used, of types of transformation, of devices for carrying out the transformations and of devices for carrying out the covering phases, it is possible, however, to identify a few characteristics common to the great majority of currently operational rapid prototyping machines.
This is because material transformations are always induced, for reasons of physical accessibility, via the so-called "free" face of the layer being treated (as opposed to the so-called "hidden" face which is in contact with the previous layer). In theory, it is possible to perform transformations over the entire surface defined by the free face, but very often the device used for carrying out the local transformations makes it possible, because of its construction, to have access only to one portion of this free face, which in the rest of the present description will be called the "working field".
Another characteristic common to the majority of rapid prototyping machines is the use of a doctor blade even in the case of machines using the technique for agglomerating powders or for cutting sheets of paper. As a general rule, the doctor blade is in the form of an assembly of mechanical components of elongate shape, generally approximately rectilinear, which may or may not perform their own movement, coming into direct or indirect contact (for example by brush elements) with the raw material lying near the free surface. The assembly is set in motion by a motor-driven device approximately parallel to the free surface of the raw material. In order to put a new layer in place over the entire working field, it is necessary to make the doctor blade follow a path whose amplitude is sufficient to sweep over the major part of the working field.
The transformation-inducing device does not always allow electromagnetic radiation or particles to be emitted simultaneously in the direction of the set of points making up the working field. This is because, for example in the case of a device using a laser beam coupled to a deflection system by galvanometer mirrors, the transformations are carried out by directing the movement of the point of impact of the laser beam on the working field in order to produce a line of successive vectors, the set of which corresponds to the geometry of the cross section to be transformed. In contrast, when a device based on the principle of modern video projectors, i.e. consisting of a lamp, an optical device and a dynamic mask, is used, it is possible to illuminate the entire working field simultaneously as long as, however, the mechanical elements of the doctor blade do not act as an obstacle to the incident photons, in which case a shadow is cast on the working field. This shadow effect, although fortuitous, runs the risk of disturbing the action of the transformation-inducing device, even in the case of the line of successive vectors, with the consequence of creating substantial defects in the components produced. However, in most machines, all the degrees of freedom of the doctor blade and of the transformation-inducing device are such that suitable synchronization of the control of these members is essential in order to avoid damage likely to occur because of the shadow effect. When it is particles of matter which are emitted, there isn't a real "shadow" effect in the normal meaning of the term, nevertheless, for reasons of uniformity in the terminology, we will continue in the rest of the description to speak of the constraint due to the "shadow effect" even if the doctor blade does not, in this case, constitute an obstacle to light radiation but an obstacle to a flux of matter particles.
U.S. Pat. No. 5,204,823 describes a rapid prototyping machine which eliminates the shadow effect, by rendering the doctor blade and the device for carrying out transformations integral. However this solution means that the transformation-inducing device has to be mobile, which is incompatible with (or too expensive for) most common light sources and optical systems.
The solution most widely adopted for managing the shadow effect consists of carrying out a sequential succession of transformation phases and of covering phases, the doctor blade being at rest during the transformation phases in a position such that the entire working field can be accessed by the transformations-inducing device, and no transformation taking place when the doctor blade is moving. In general, the path of the doctor blade during manufacture is a forward-and-back cycle between two rest positions which are opposite each other with respect to the working field. For each covering phase, the doctor blade performs a forward movement from one of the rest positions to the other, thus acting on the entire working field.
Such a manner of operation certainly has the advantage of being very simple to implement, but constitutes a major handicap in terms of the machine's productivity and of operating costs. This is because, despite the constant efforts by manufacturers, the maximum speed of movement of the doctor blade remains relatively low, because of constraints associated with the rheological behavior of the raw materials, which generates a minimum duration of the covering phase. This maximum speed is of the order of a few centimeters per second, which means, for example, for a square working field of 500 mm a side, a duration of each covering phase of the order of a few seconds to a few tens of seconds. However, in order to produce components having an average volume corresponding to a cube of 300 mm a side with an acceptable surface finish, thousands of layers have to be put into place and, consequently, whatever the efficiency of the transformation-inducing device, several hours of operation by machine are necessary. In addition, for example in the case of the use of lamps or lasers, the photons are emitted continuously throughout the duration of the manufacture, since the lifetime of sources of this type when operated discontinuously is greatly reduced, resulting in a much greater energy cost than that corresponding to the effective use of all the light energy.
The relative duration of the covering phases with respect to that of the transformation phases depends on many parameters (reactivity of the raw material, power of the transformation-inducing device, size of the working field, shape of the components, etc.), but as a general rule these two durations are of the same order of magnitude, which results in an energy efficiency and a productivity limited to approximately 50% of the theoretical potential. If more efficient devices are employed for carrying out the transformation phases, the maximum time saving to be hoped for in the total manufacturing time is therefore 50% (the duration of the covering phases cannot be shortened), which is relatively negligible compared with the drop in yield which results therefrom.